Process fob the production of



-July 1, i942.

G. wlDMER ETAL l 2,288,964 PROCESS FOR THE PRODUCTION OF MEL-MINE Filed Feb. 3, 1940 dal."

condensation solutions.

Patented July 7,

Gustave Widmer and Josef Jakl, Basel. and Willi Fisch, Binningen, Switserland, assignnl's, by

mesne assignments, to Ciba Products tion. Dover, Del.

Corpora- Appcation February 3, i940, Serial No. 3117.193 In Switzerland March 30, 1939 Melamine which has been manufactured by the usual technical p is not chemically pure. For example, the crude melamine obtained from dicyandiamide and ammonia in the autoclave, in spite of the fact that it contains about 97% -pure melamine, has a light gre'yish to greybrown color, and yields with formaldehyde condensation solutions whichare colored more or less brownish-yellow, and which, in spite of the use of decolorizing agents, are unsuitable for many purposes. An endeavour has therefore been made to discover a simple method of purifying crude melamine and other raw materials which contain melamine, which will permit the manufacture of practically pure melamine on the technical scale.

The most important known method of purification is that of crystallization. Liebig (Annalen der Pharmacie, vol. l0, page 19, 1934) has described melamine as being soluble with difficulty in cold water but more readily soluble at the boil. It is actually possible, particularly by the aid of decolorizing carbon, to obtain a product which yields light-colored topractically colorless I'he disadvantage of this process is the large quantity of water necessary for the recrystallization oflarge quantities l of crude melamine. s

Sublimation may be mentioned as a further method of puriilcation. In this connection, Drechsel (Journal fr prakt. Chemie, vol. 13, page 332, 1876), states thatmelamine sublimes unchanged in the form of magnificent small crystals, when it is heated sufllciently strongly but not allowed to melt. By this method, Drechsel was able to sublime more than a gram of melaabove its Vmelting point has not *yet been moreexactly described. Liebig, in thelarticle mentioned above, states "On heating, the crystals decrepitate and melt to form a transparent liquid, which solidifles in a crystalline state. on cooling. On heating more strongly, the melted substance is drawn up the sidesof the tube without subliming: if a portion of it flows on to a glowing part of the glass, it is decomposed with liberation of ammonia, leaving a lemon-yellow colored residue which, on heating to a glow, disappears completely with the formation of cyanogen and nitrogen." Drechsel confirms these statements of Liebig in his article above quoted. Ostrogovich (Gazetta Chimica Italiana, vol. 65, page 577, 1937), states that melamine, in the pure state,-

' it melts immediately, volatilizesin the form of ing necessary causes decomposition of a portion of the melamine, and the yield of sublimed melamine drops below 50%.

If crude melamine be heated in the form of a thin layer'to temperatures which lie below its melting point, for example, to 30G-340 C., a slow evaporation, with the formation of fumes, takes place, leaving as ilnal residue a light, voluminous powder.

A rather more favorable result is obtained by sublimation in a vacuum, particularly if smaller quantities are used. The sublimation, however, takes place only slowly, which causes a partial a white, microcrystalline vapor and may be obtained in a chemically pure state if it be removed from the heating lzone in a suitable way, for instance b'y means of a gas stream. This is the more surprising insofar as it could not be forcseen that no appreciable decomposition of the melamine would take place.

The melamine obtained in this manner may be shown by analysis to theoretical values.

The small remaining residue is a yellow-brown mass, which consists, inl the main, of the deamidation products of melamine, such as melam, melem. and'mellone.

The process may be carried out in such a manner, for example, that melamine is continuously drawn on to a heated surface, which is prefer-y ably heated to a temperature of about 400 C.- for example, a rotating electric hot plate may be used. The escaping melamine is removed from` the heating zone by means of a current of gas and is caught in suitable containers-for expossess the calculated ample, lter bags, dust chambers, etc. It is a white, very light powder, which dissolves very rapidly in water and in aqueous formaldehyde on account of its line state of division. It yieldsa f solution which is practically( colorless.

The temperature range of the heating surface which is preferable for thisprocess lies between .l

350 and 450- C. v

' Example 3400 of crude melamine '(97%) are poured,

` a little at a time, into the feeding funnel (marked A in the accompanying drawing) and,I by means of the carrier band B, are fed continuously in a thin stream through the charging funnel C on to the rotating electrically heated plate D, which is maintained at about 400 C. and is driven byv the transmission K. Charging may be regulated. either by the quantity of crude melamine fed into the apparatus or by the speed of rotation oi' the hot plate. Immediately on contacting the hot plate, the melamine melts and escapes instantaneously in the form of a white vapor, consisting of pure, micro-crystalline melamine. It escapes through a wide,outlet tube-F, which fits exactlyover the hot plate, and which is connected at the other end with a large bag of ilterv cloth G which, in its turn, connectswith a collecting reservoir H below. vThe bag is contained in a closed chamber J in which a slightly reduced pressure is maintained. In this manner, the necessary air current is attained, which draws the volatilized melamine into the bag: this air current is to be regulated insuch a manner that only melamine-and lnot flying dust, originating from the residue which remains on the *hot plateis transferred into the bag.. 'Ihe residue is continuously removed from the hot plate by.

means of a scraper E and should'vaccumulate in thin leaflets, having a yellow-brown color. experimentdescribed above, using a hot plate of 22 cm. diameter and one feeding system only for The` I scale.

the vcrude melamine, should require about 3 hours for its completion. The yield of sublimed pure melamine amounts to- 3100 gms., representing 591% ofthe crude melamine used.

By increasing the size of the heating surface of the hot plate and by using more than one feeding'A system, purlcation of the crude inelamine may easily be undertaken on a technical Instead of using a reduced pressure in the enclosed chamber J to remove the pure melamine formed, the charging lside of the apparatus may be enclosed, and work maybe carried out with slightly increased pressure on this side, using air or another gassuch as nitrogen, hydrogen, etc..

vas the conveying gas.

What we claim'is: l. Process for the preparation of pure melai mine from raw materials containing melamine,

ing melamine on to heated surfacesv which are heated to a temperature between 350 and 450 C., in such a manner that immediate volatilization of .the melamine takes place and' the melamine vapor thus formed is removed from the heating zone by. means of a current of gas and is collected in a suitable manner.

` GUsTAvE WIDMER.

JOSEF JAKL. wnm FISCH. 

